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How would you implement the Builder pattern using method chaining (a fluent API) to construct a complex object step by step?

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Answer

Define a builder class with methods that each set part of the object being built and RETURN 'this' (the builder itself), enabling method chaining; a final Build() method assembles and returns the fully-configured object — useful when an object has many optional configuration parameters that would otherwise require an unwieldy constructor with many arguments.
public class EmailBuilder {
  private readonly Email _email = new();
  public EmailBuilder To(string address) { _email.To = address; return this; }
  public EmailBuilder Subject(string subject) { _email.Subject = subject; return this; }
  public Email Build() => _email;
}

var email = new EmailBuilder().To("sam@x.com").Subject("Hello").Build();
Real-world example Constructing a complex configuration object (like an HTTP request or an email message) with many optional parameters readably.

Common follow-ups: How do C# 12's collection expressions or object initializer syntax sometimes reduce the NEED for a full Builder pattern for simpler cases?

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All 11 Design Patterns in C# questions